The present invention relates to a process for preparing aldehydes by hydroformylation of olefins.
More particularly, the invention relates to a process for catalytically hydroformylating olefins such as propylene and 1-pentene with hydrogen and carbon monoxide in the presence of a complex comprising platinum and a bidantate ligand to synthesize selectively corresponding aldehydes such as butylaldehyde and hexylaldehyde.
Hydroformylation reaction of olefins with hydrogen and carbon monoxide has been known for a long time. As catalysts for this hydroformylation reaction, there have heretofore been used cobalt-carbonyl complexes (catalysts of the first group) such as dicobalt octacarbonyl, tetracarbonyl cobalt hydride and organic phosphine-substituted cobalt carbonyl, rhodium-carbonyl complexes (catalysts of the second group) such as Rh.sub.4 (CO).sub.12, RhCl(CO)(PPh.sub.3).sub.2 and HRh(CO)(PPh.sub.3).sub.3 and platinum-phosphine complex-tin chloride catalysts (catalysts of the third group).
Catalysts of the first group are relatively low in the catalytic activity, and therefore, they cannot be used unless the temperature is elevated to a considerably high level. It is known that catalysts of the second group have a relatively high activity, but they are defective in that, in the reaction of olefins such as 1-pentene as the starting substance, the linear isomer/branched isomer ratio in the reaction product is low. Accordingly, in order to obtain a linear isomer which is industrially important, it is necessary to have a large amount of a phosphine ligand present in the reaction system. Catalysts of the third group show only an intermediate activity between those of the above-mentioned rhodium and cobalt catalysts, and when a bidantate ligand such as Ph.sub.2 P(CH.sub.2).sub.2 PPh.sub.2 is employed, the aldehyde-forming activity is drastically lowered.
We conducted research directed to developing a catalyst having a higher activity for hydroformylation of olefins than conventional rhodium catalysts and being capable of providing a product having a higher linear isomer/branched isomer ratio. As a result, to our great surprise, it was found that when among bidantate ligands which have been considered unsuitable as promoters for platinum catalysts, a specific ligand is selected and used in combination with a platinum catalyst, the activity of the platinum catalyst is substantially enhanced over the activity of a rhodium carbonyl complex such as RhCl(CO)(PPh.sub.3).sub.2 or HRh(CO)(PPh.sub.3).sub.3 and an aldehyde mixture containing a linear isomer at a very high content ratio can be obtained as the reaction product. Based on this finding, we have now completed the present invention.